In optical scanning systems of the kind in which the scanning elements used either have the form of rotating mirror polygons, or oscillation mirrors, it is desirable (a) to reduce dimensions to the greatest possible extent, so as to reduce inertia during oscillatory motion of the mirrors, (b) to reduce as far as possible rotational or oscillatory speeds, and (c) to achieve a scanning rate which is convertible with the line sweep of a conventional television system. In the case of these two latter conditions, it is attempted, when using rotating facet-polygons to place the greatest possible number of mirroring facets around the periphery of the drum, since television systems have a relatively high line frequency. Unfortunately, it is not possible to include as many facets as might be desired since efficiency decreases with increasing numbers of facets and since the performance of the scanning arrangement or device will fall. A factor of merit in respect of an optical scanner, e.g., for a line scan, is the optical invariant E. The optical invariant is a system constant which at the exit pupil of the scanner can be expressed as E=.0..times..alpha., where .0. is the aperture of the scanner arrangement, i.e., the factor determining the cross-section of the beam passed through the system, and .alpha. is the field angle of the scan. The formula for calculating invariance is applicable only when the object scanned is located in air (n=1). In the case of intermediate imaging in the system, the expression takes the form E=u.times.L.times.n, where L is the length of the line scan, u is the aperture angle at the intermediate image, and n is the refractive index of the medium where imaging takes place. Expressed more simply, it is possible, for example, in the case of a scanner constructed for infrared light with a given invariant E, to exchange thermal sensitivity for geometric resolution and vice versa, i.e., the following relationship prevails: E.about.element per line/NETD, where NETD is the so-called Noise Equivalent Temperature Difference, which is a measurement of the responsiveness or sensitivity of the system.
This means that, with a given, specific optical invariant in respect of an optical scanning system, it is always necessary to compromise between image resolution and sensitivity. On the other hand, an increase in the optical invariant heightens the possibility of improving both of these properties. Consequently, attempts are made to improve the optical invariant, in order to thus improve the performance of the system.
When, for example, a line scan is effected with the aid of a rotating drum provided with mirroring facets around its periphery, such as to form a mirror polygon, it can be shown that the following relationship prevails in air upon reflection against the drum: ##EQU1## where D is the drum diameter, N is the number of facets present, and .eta. is the scan efficiency, i.e., the relationship between the useful part of the line scan over a facet and the whole scan over the facet. It will be seen from this that the invariant decreases with the square of the number of facets provided. This decrease of the invariant is unfavorable, due to the desirability of incorporating as large a number of facets as possible, as mentioned above. Performance should therefore be increased in one way or another, in order to compensate for the unfavorable effect created by a large number of facets.
By permitting the scanning function to take place through reflection within a refractive material, it is possible to increase performance by a factor corresponding to the refractive index of said material, in comparison with corresponding scanning devices with reflection in air, i.e., ##EQU2## where n is the refractive index of the refractive material. For example, when germanium is used the gain is a factor of 4, while the gain afforded by silicon is a factor of 3.4. Other materials of high refractive index capable of providing an improved result are also to be found, such as cadmium telluride with n.apprxeq.2.7 or zinc selenide with n.apprxeq.2.4.
The above discussion has been made in respect of an optical scanning device in the form of a rotating drum provided with mirroring or reflecting facets. A similar discussion can also be made, however, in respect of an optical scanning device in the form of an oscillating mirror.